This disclosure relates in general to a method and a system for directionally drilling a borehole with a rotary drilling system. More specifically, but not by way of limitation, an embodiment of the present invention provides for controlling motion of the rotary drilling system in the borehole when a side force is applied to the drilling system so that the drilling system directionally drills the borehole through an earth formation. In certain aspects of the present invention, side cutting of a sidewall of the borehole by a drill bit under an applied side force is controlled by a geostationary element to provide for directional side cutting and, as a result, directional drilling of the borehole through the earth formation.
In many industries, it is often desirable to directionally drill a borehole through an earth formation or core a hole in sub-surface formations in order that the borehole and/or coring may circumvent and/or pass through deposits and/or reservoirs in the formation to reach a predefined objective in the formation and/or the like. When drilling or coring holes in sub-surface formations, it is sometimes desirable to be able to vary and control the direction of drilling, for example to direct the borehole towards a desired target, or control the direction horizontally within an area containing hydrocarbons once the target has been reached. It may also be desirable to correct for deviations from the desired direction when drilling a straight hole, or to control the direction of the hole to avoid obstacles, such as formations with adverse drilling properties. It should be understood that drilling of boreholes may comprise vertical drilling, horizontal drilling and angled drilling and many drilling jobs may include combinations thereof.
In the hydrocarbon industry for example, a borehole may be drilled so as to intercept a particular subterranean-formation at a particular location. In some drilling processes, to drill the desired borehole, a drilling trajectory through the earth formation may be pre-planned and the drilling system may be controlled to conform to the trajectory. In other processes, or in combination with the previous process, an objective for the borehole may be determined and the progress of the borehole being drilled in the earth formation may be monitored during the drilling process and steps may be taken to ensure the borehole attains the target objective. Furthermore, operation of the drill system may be controlled to provide for economic drilling, which may comprise drilling so as to bore through the earth formation as quickly as possible, drilling so as to reduce bit wear, drilling so as to achieve optimal drilling through the earth formation and optimal bit wear and/or the like.
One aspect of the drilling process may be referred to as “directional drilling.” Directional drilling is the intentional deviation of the borehole/wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction.
Directional drilling may be advantageous in situations such as offshore drilling or the like because it may enables a plurality of wells to be drilled from a single drilling platform. Directional drilling may also enable horizontal drilling through a reservoir to provide for enhanced exposure of a borehole to the reservoir, i.e., horizontal drilling enables may provide for a longer length of the wellbore to traverse the reservoir, thus increasing the production rate from the well.
A directional drilling system may also be useful in vertical-type drilling operation. For example, in a vertical drilling operation, the drill bit may veer off of a planned vertical drilling trajectory because of the unpredictable nature of the formations being penetrated and/or the varying forces that the drill bit experiences. When such a deviation occurs, a directional drilling system may be used to put the drill bit back on a vertical course.
The monitoring process for directional drilling of the borehole may include determining the location of the drill bit in the earth formation, determining an orientation of the drill bit in the earth formation, determining a weight-on-bit of the drilling system, determining a speed of drilling through the earth formation, determining properties of the earth formation being drilled, determining properties of a subterranean formation surrounding the drill bit, looking forward to ascertain properties of formations ahead of the drill bit, seismic analysis of the earth formation, determining properties of reservoirs etc. proximal to the drill bit, measuring pressure, temperature and/or the like in the borehole and/or surrounding the borehole and/or the like. In any process for directional drilling of a borehole, whether following a pre-planned trajectory, monitoring the drilling process and/or the drilling conditions and/or the like, it is necessary to be able to steer the drilling system.
Forces which act on the drill bit during a drilling operation include gravity, torque developed by the bit, the end load applied to the bit, and the bending moment from the drill assembly. These forces together with the type of strata being drilled and the inclination of the strata to the bore hole may create a complex interactive system of forces during the drilling process.
In many applications, the drilling system may comprise a “rotary drilling” system in which a downhole assembly, including a drill bit, is connected to a drill string (casing string) that may be driven/rotated from the drilling platform. In a rotary drilling system, directional drilling of the borehole may be provided by varying factors such as weight-on-bit, the rotation speed, etc.
With regards to rotary drilling, known methods of directional drilling include the use of a rotary steerable system (“RSS”). In an RSS, the drill string is rotated from the surface causing the drill bit to rotate against an earth formation at the end of the borehole and to drill through the formation. Downhole devices and systems, such as discussed below, may be activated to cause the drill bit to drill in a desired direction. Rotating the drill string greatly reduces the occurrences of the drill string getting hung up or stuck during drilling.
Rotary steerable drilling systems for directionally drilling boreholes through the earth may be generally classified as either “point-the-bit” systems or “push-the-bit” systems. In both types of system the goal of the system is to apply a side force to the drill bit so as to cause the drill bit to drill directionally, i.e., off axis.
In the point-the-bit system, the axis of rotation of the drill bit is deviated from the local axis of the bottomhole assembly (“BHA”) in the general direction of the new hole. The term bottomhole assembly may be used to refer to the plurality of devices/systems attached to the drill string in the borehole. As such, the bottomhole assembly may comprise the drill bit, a drill collars, gauge pads, bit sub, a mud motor, stabilizers, heavy-weight drillpipe, jarring devices (“jars”) and crossovers and/or the like.
In general, the borehole may be propagated/drilled in accordance with the customary three-point geometry defined by upper and lower stabilizer touch points and the drill bit. The angle of deviation of the drill bit axis coupled with a finite distance between the drill bit and lower stabilizer results in a non-collinear condition required for a curve to be generated. There are many ways in which this may be achieved including a fixed bend at a point in the bottomhole assembly close to the lower stabilizer or a flexure of the drill bit drive shaft distributed between the upper and lower stabilizer. By managing the stabilizer positions, introduction of an angled sub between the stabilizers, the amount of flexibility and location of a flexure and/or the like directional drilling may be attained, i.e., a side force may generated on the drill bit causing off axis drilling of the borehole.
Pointing the bit may comprise using a downhole motor to rotate/move the drill bit in the borehole. For example, the drill bit, the motor and drill bit may be mounted upon a drill string that includes an angled bend and the motor may rotate the drill bit tom provide that the angles bend causes a side force to be applied to the drill bit and, consequently, the drilling of the borehole by the drill bit in the direction of the side force. In such a system, the drill bit may be coupled to the motor by a hinge-type or tilted mechanism/joint, a bent sub or the like, wherein the drill bit may be moved so that is inclined relative to the motor. When variation of the direction of drilling is required, the rotation of the drill string may be stopped and the bit may be positioned in the borehole, using the downhole motor, so that the drill bit in the required direction and rotation of the drill bit may start the drilling in the desired direction. In such an arrangement, the direction of drilling is dependent upon the angular position of the drill string.
In its idealized form, in a pointing the bit system, the drill bit is not required to cut sideways because the bit axis is continually rotated in the direction of the curved hole. Examples of point-the-bit type rotary steerable systems, and how they operate are described in U.S. Patent Application Publication Nos. 2002/0011359; 2001/0052428 and U.S. Pat. Nos. 6,394,193; 6,364,034; 6,244,361; 6,158,529; 6,092,610; and 5,113,953 all herein incorporated by reference.
Push the bit systems and methods make use of application of force against the borehole wall to bend the drill string and/or direct application of a side force on the drill bit to drill in a preferred direction. In a push-the-bit rotary steerable system, the requisite non-collinear condition is achieved by causing a mechanism to apply a force or create displacement in a direction that is preferentially orientated with respect to the direction of hole-propagation. There are many ways in which this may be achieved, including non-rotating (with respect to the hole) displacement based approaches and eccentric actuators that apply force to the drill bit in the desired steering direction. Again, steering is achieved by creating non co-linearity between the drill bit and at least two other touch points. In its idealized form the drill bit is required to cut sideways in order to generate a curved hole. Examples of push-the-bit type rotary steerable systems, and how they operate are described in U.S. Pat. Nos. 5,265,682; 5,553,678; 5,803,185; 6,089,332; 5,695,015; 5,685,379; 5,706,905; 5,553,679; 5,673,763; 5,520,255; 5,603,385; 5,582,259; 5,778,992; 5,971,085 all herein incorporated by reference.
Known forms of RSS may include a “counter rotating” mechanism, which rotates in the opposite direction of the drill string rotation. Typically, the counter rotation occurs at the same speed as the drill string rotation so that the counter rotating section maintains the same angular position relative to the inside of the borehole. Because the counter rotating section does not rotate with respect to the borehole, it is often called “geostationary” by those skilled in the art. In this disclosure, no distinction is made between the terms “counter rotating” and “geo-stationary.”
A push-the-bit system typically uses either an internal or an external counter-rotation stabilizer. The counter-rotation stabilizer remains at a fixed angle (or geo-stationary) with respect to the borehole wall. When the borehole is to be deviated, an actuator that is held geostationary by the stabilizer may be actuated to press a pad against the borehole wall in the opposite direction from the desired deviation. The result is that a side force is applied to the drill bit that pushes the drill bit to cut in the desired direction.
The force generated by the actuators/pads is balanced by the force to bend the bottomhole assembly, and the force is reacted through the actuators/pads on the opposite side of the bottomhole assembly and the reaction force acts on the cutters of the drill bit, thus steering the hole. In some situations, the force from the pads/actuators may be large enough to erode the formation where the system is applied.
For example, the Schlumberger™ Powerdrive™ system uses three pads arranged around a section of the bottomhole assembly to be synchronously deployed from the bottomhole assembly to push the bit in a direction and steer the borehole being drilled. In the system, the pads are mounted close, in a range of 1-4 ft behind the bit and are powered/actuated by a stream of mud taken from the circulation fluid. In other systems, the weight-on-bit provided by the drilling system or a wedge or the like may be used to orient the drilling system in the borehole.
Another way to generate a side force on the drill bit is to use a drill bit with a cutter structure that is designed to generate a varying or relatively constant side force on the drill bit in a direction that remains broadly fixed in one direction relative to the body of the drill bit (or at least in one quadrant). This may be readily achieved by judicious arrangement of the cutters as is done in the case of the anti-whirl bit (where the mean cutting side force is directed towards a particular gauge pad which is thereby held, while rotating, against the bore wall). For directional drilling purposes, the off-center side force developed by the cutter arrangement may be used to drive a set of gauge cutters towards the borehole wall (the opposite of anti-whirl). As the bit rotates, so does the cutting side force and, therefore, the preferred cutting direction.